The St. Francois Mountains of southern Missouri is an area that has drawn igneous petrologist from all over the world to its location. It is comprised of exposed Precambrian rocks brought to the surface by volcanic and intrusive activity that is dated to be 1.45-1.28 billion years in age. Regionally speaking, the rocks of the St. Francois Mountains are a part of the Mesoproterozoic Easter Granite-Rhyolite Province. It is common for these rocks to be buried underneath Phanerozoic sediments and have only been study through the process of drilling cores. When discussing the possibilities of where the magma of the Eastern Granite-Rhyolite Province of the St. Francois Mountains is derived form, two tectonic environments have been hypothesized.
Subduction Zone Hypothesis
To aid in the hypothesis, it is best to conduct tests on contemporaneous basalts from the Granite-Rhyolite Province due to the accuracy of geochemical evidence for the tectonic setting.
Looking at the Silver Mine rocks of the St. Francois Mountains; the trace element geochemistry suggest that a subduction zone or continental margin best explains this formation. The low Th contents and the Th/La ratio of the Silver Mine rocks support evidence that their parental magmas experienced minimal contamination from continental crust. It is concluded that many of the important trace elements of the Silver Mine basalts clearly fall within a subduction zone and are suggestive of an active continental margin. Trace elements suggest that the subduction modified lithospheric mantle contributed significantly to the mantle source of the early erupted lavas.
To add to this hypothesis, it is believed that an environment in which silicic magmatism is followed by post-orogenic lithospheric thinning from a previously active continental margin, is a more viable theory for an anorogenic model for the Mesoproterozoic mid-continental magmatism. Lithospheric thinning brings hot asthenosphere into closer contact to the lower crust and eventually gives way to widespread production of the silicic magmas with anorogenic affinities. It is proposed that due to the given geochemical evidence, subduction was taking place at the St. Francois Mountains and the region was then located in a continental back arc basin setting. Extension associated with basin allowed for the generation of basic magma, with most of the magma being stored in the base of the crust which would then provide the input needed for softening and later crustal extension and igneous production.
Early Mesoproterozoic crustal development in North America involved the production of large volumes of A-type felsic rocks forming the Granite-Rhyolite Province. The North American Mesoproterozoic Granite Rhyolite Province, which consist of felsic rocks that are metaluminous to peraluminous, may be related to anorogenic process. The felsic volcanic rocks have yielded igneous crystallization ages of 1.45 – 1.50 Ga by U-Pb zircon dating and because of this the alkaline A-type chemistry of the 1.48Ga suite as well as the lack of metamorphism and deformation have led to this vent being classified as anorogenic. The chemical composition of the rhyolites is consistent with their formation by fractional crystallization primarily of plagioclase and K-feldspar, from dacite or less siliceous melt. The parental magma was probably not much less siliceous than dacite because there are no 1.48 Ga igneous rocks less siliceous than 67% SiO2. The magmas must have been formed primarily by partial melting of pre-existing crustal rocks. Partial melting of tonalitic to granodioritic crust will produce the calc-alkaline granitoids that are metaluminous at low pressures and peraluminous at high pressures. Calc-alkaline chemical affinities may be the result of crustal contamination by a source with a bulk intermediate to felsic composition. This type of partial melting can be further broken down into a subcategory known as A2-type granites.
When considering the evidence of metaluminous to mildly peraluminous compositions and the lack of metamorphism and deformation, these characteristics suggest that the St. Francois Mountains rhyolites formed by partial melting of the contemporaneous calk-alkaline rocks. This is indicative of an extensional setting within the continental plate overlaying an active subduction zone.
This hypothesis can be verified if an assumption is made that the Granite Rhyolite Province was situated in a continental back-arc region in which back-arc extension was the driving force for generating the basaltic melts. It is possible that large volumes of basic material would not be erupted, but rather stored at the base of the crust. This magma would then be able to provide the heat source needed to cause crustal melting of the calc-alkaline crust producing the relatively large volumes of metaluminous to peraluminous A2-type felsic volcanism. Magmatism in this case is directly linked to subduction zone processes. Shows a relationship between Y/Nb that reflects crustal contamination of magma due to continental crust trends having a Y/Nb >1.
An important conjecture to note is that it has been discovered that early mafic lavas from the nearby Neoproterozoic Midcontinental Rift also share very similar geochemical analysis with the St. Francois Mountains Basalts. The Neoproterozoic Midcontinental Rift is apart if the flood basalt province that is linked to a mantle plume. Trace Elements suggests that the subduction modified lithosphere contributed significantly to the mantle source of the Neoproterozoic Midcontinental Rift. This subduction lithosphere contamination is thought to be the reason why we see two incompatible parental source magma types having the same geochemistry.
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Mantle Plume Hypothesis
A hypothesis of rifting has been formulated involving a mantle plume, or a mantle superswell that has developed underneath a supercontinent and is adding an extensional collapse following a convergent margin orogenesis contributing to accretion and crustal thickening.
Magmatic complexes in the buried Precambrian basement of the Midcontinent indicate a period of widespread anorogenic magmatism that may have been induced by intraplate hot-spot activity. The post-Precambrian geologic record of the region suggests that recurrent, but localized and less intense, hot-spot activity was responsible for emplacements of alkaline ultramafic rocks, for tectonic adjustments,
A mantle plume or superswell would support evidence of a heat source for the widespread crustal fusion and silica magma production in the province, however it would also yield a large volume of mafic igneous rocks. Looking closely at the Silver Mines of the St. Francois Mountains, mafic igneous rocks are only a minor component of the Granite-Rhyolite Province. This means that a mantle plume or superswell is very doubtful.
Adding to this doubt is the scarcity of the 1.48Ga old basaltic rocks and the absence of high temperature basic rocks. The thermal anomaly associated with a plume originating at the boundary between the upper and lower mantle would be no more than 300 km in diameter and therefore too small to account for the Granite Rhyolite Province. There is also no evidence for doming of the crust immediately prior to magmatism as would be expected of a mantle plume impinged on the lithosphere.
Conclusion
The most likely source of the parental magma deriving the Eastern-Granite-Rhyolite Province of the St. Francois Mountains was subduction of an active continental margin. The biggest evidence to support this theory is the geochemical analysis of the A2-type granites found in the province which are metaluminous to peraluminous and indicative of a subduction zone.
The parental magma was formed from partial melting of pre-existing crustal rock with calc-alkaline chemical affinities contributing to the crustal contamination of the magma. The contamination is evidence of an extensional setting and is assumed that the province is in a back-arc extension with the magma being stored at the base of the crust, causing the partial melting needed for igneous rock production.
A theory was proposed that that rifting from a mantle plume could be the source of parental magma for the province, especially since the neighboring Neoproterozoic Midcontinental Rift Province shows trace element geochemistry for this kind of activity, as well as the idea that a mantle plume would support the heat source needed for widespread silica magma production in the province. Unfortunately, this idea environment would also yield a large volume of mafic igneous rocks that is not seen in the field area. Assisting to the idea that it is not a mantle plume is also the evidence of a scarcity of 1.48Ga basaltic rocks the complete absence of high temperature basalts too. Lastly, there is also no evidence of doming of the crust immediately prior to the magmatism, as would be expected of a mantle plume.
Subduction zone partial melting magmatism is the only scenario in which reputable evidence with lab experimentation and field observations can provide insight into the possible source of parental magmatism of the St. Francois Mountains.
References
- Walker, J.A., Pippin, C.G., Cameron, B., and Patino, L., 2002, Tectonic Insights Provided by Mesoproterozoic Mafic Rocks of the St. Francois Mountains, Southeastern Missouri: Precambrian Research, v. 117, p. 251–268, doi: 10.1016/s0301-9268(02)00091-8.
- Menuge, J.F., Brewer, T.S., and Seeger, C.M., 2001, Petrogenesis of Metaluminous A-Type Rhyolites from the St. Francois Mountains, Missouri and the Mesoproterozoic Evolution of the Southern Laurentian Margin: Precambrian Research, v. 113, p. 269–291, doi: 10.1016/s0301-9268(01)00211-x.
- Nabelek, P.I., and Russ-Nabelek, C., 1990, The Role of Fluorine in the Petrogenesis of Magmatic Segregations in the St. Francois Volcano-Plutonic Terrane, Southeastern Missouri: Geological Society of America Special Papers Ore-bearing Granite Systems; Petrogenesis and Mineralizing Processes, p. 71–88, doi: 10.1130/spe246-p71.
- Sylvester, Paul Joseph, 'Geology, Petrology, and Tectonic Setting of the Mafic Rocks of the 1480 MA Old Granite-Rhyolite Terrane of Missouri, USA' (1984). Retrospective Theses and Dissertations. 62. https://openscholarship.wustl.edu/etd_restrict/62
- Frost, C.D., and Frost, B.R., 2013, Proterozoic Ferroan Feldspathic Magmatism: Precambrian Research, v. 288, p. 151–163.
- Kisvarsanyi, E.B., 1980, Granitic Ring Complexes and Precambrian Hot-Spot Activity in the St. Francois Terrane, Midcontinent Region, United States: Geology, v. 8, p. 43, doi: 10.1130/0091-7613(1980)82.0.co;2.
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